Fix extraneous 'static'

Fixes #717

README.rst

@@ -174,6 +174,7 @@ Inspired by `Keepachangelog.com <http://keepachangelog.com/>`__.
 - Unreleased - Breathe v4.31.0
 
   - Collapse multiple retvals into a single bullet list. #697
+  - Fix duplicate ``static`` in function declarations. #717
 
 - 2021-05-06 - Breathe v4.30.0
 

breathe/renderer/sphinxrenderer.py

@@ -1851,11 +1851,13 @@ def visit_function(self, node) -> List[Node]:
                     elements.append('virtual')
                 if node.explicit == 'yes':
                     elements.append('explicit')
-                if 'constexpr' in dir(node):
-                    assert node.constexpr == 'yes'
-                    elements.append('constexpr')
-                elements.append(''.join(n.astext()
-                                        for n in self.render(node.get_type())))  # type: ignore
+                # TODO: handle constexpr when parser has been updated
+                #       but Doxygen seems to leave it in the type anyway
+                typ = ''.join(n.astext() for n in self.render(node.get_type()))
+                # Doxygen sometimes leaves 'static' in the type,
+                # e.g., for "constexpr static auto f()"
+                typ = typ.replace('static ', '')
+                elements.append(typ)
                 elements.append(name)
                 elements.append(node.get_argsstring())
                 declaration = ' '.join(elements)